Don’t try to parse the shortcodes and then regex the HTML. Use get_shortcode_regex() to parse the raw post content:
$content = $content_post->post_content;
preg_match_all("/$pattern/",$content,$matches);
Then crawl $matches to find your shortcode data. Use shortcode_parse_atts($matches[3][0]) (note $matches[3][0] to give the first element within the matched shortcode’s attributes) to pull apart the attribute string.
Reference:
https://wordpress.stackexchange.com/a/73461/21376